These studies are designed to test whether individual differences observed in the response to overfeeding or aerobic exercise-training are associated with heredity. Two major experiments will be conducted each on 12 pairs of adult male monozygotic (MZ) twins. The experimental treatment will last for 100 days, in each case, and will be preceeded by a week of observation to establish body composition, habitual energy intake, energy cost of habitual activities and other parameters. In the first experiment, both members of each MZ pair will be submitted to an overfeeding treatment of an extra 1000 kcal/day over their habitual energy intake. In the second experiment all twins will be submitted to a negative energy balance of 1000 kcal/day induced by a program of exercise. Treatment in both cases will be applied in cycles of 6 consecutive days followed by a day of rest. Changes in body density, total body fat, subcutaneous extremity and trunk fatness, fat cell size and fat cell lipolysis, lipogenesis and lipoprotein lipase activity from two sites of biopsy (suprailiac and femoral), blood lipids and lipoproteins, resting metabolic rate, diet induced thermogenesis, exercise tolerance, glucose tolerance and their associated blood glucose, insulin, glucagon, catecholamines and thyroid hormones variations and the thyrotropes responsiveness to thyrotropin releasing hormone will be studied before and after the 100 days of treatment as well as after 25 and 50 days. All diets will be of the mixed type with about 50% carbohydrate, 35% fat, 15% protein. The aerobic exercise program will be of moderate intensity (from 40% to 70% of maximal heart rate reserve) and of sufficient duration to meet the energy cost requirement of the study. These experiments will allow the quantification of the mean response and of the inter-individual differences to overfeeding or aerobic exercise-training. Within pair resemblance and between pairs variation will also be analyzed to establish whether the adaptive changes brought about by the treatments are dependent or independent of the genotype of the individuals. Moreover, the research will make possible the comparison of short term adaptive changes with later adaptations and to test whether the pattern of the response over time is genetically determined. Results of these studies should explain some of the unaccounted variations in body composition and metabolism observed during weight reduction and weight gain.